linux-kernel - Re: [PATCH linux-next] mqueue: fix IPC namespace use-after-free

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Date:   Tue, 19 Dec 2017 11:48:19 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Giuseppe Scrivano <gscrivan@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, alexander.deucher@....com,
        broonie@...nel.org, chris@...is-wilson.co.uk,
        David Miller <davem@...emloft.net>, deepa.kernel@...il.com,
        Greg KH <gregkh@...uxfoundation.org>,
        luc.vanoostenryck@...il.com, lucien xin <lucien.xin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Neil Horman <nhorman@...driver.com>,
        syzkaller-bugs@...glegroups.com,
        Vladislav Yasevich <vyasevich@...il.com>
Subject: Re: [PATCH linux-next] mqueue: fix IPC namespace use-after-free

On Tue, Dec 19, 2017 at 11:14:40AM +0100, Giuseppe Scrivano wrote:
> mqueue_evict_inode() doesn't access the ipc namespace if it was
> already freed.  It can happen if in a new IPC namespace the inode was
> created without a prior mq_open() which creates the vfsmount used to
> access the superblock from mq_clear_sbinfo().
> 
> Keep a direct pointer to the superblock used by the inodes so we can
> correctly reset the reference to the IPC namespace being destroyed.
> 
> Bug introduced with 9c583773d03633 ("ipc, mqueue: lazy call
> kern_mount_data in new namespaces")

And just what will happen in the same scenario if you mount the damn
thing in userland without ever calling mq_open(), touch a file there,
then unmount and then leave the ipc namespace?